1. Field of the Invention
The invention relates generally to memory structures, and more particularly to the design, fabrication, and use of memory structures that combine embedded flash memory and Physical Programmable Read-Only Memory (PPROM) structures.
2. Background of the Invention
Because of its small size and low cost, PPROM Technology is used in many conventional memory applications. In order to further increase the density of PPROM memory devices, three-dimensional PPROM structures can be used. In a three-dimensional PPROM structure, layers of PPROM cells are stacked on top of each other. In general, several techniques can be used to create three-dimensional PPROM structures; however, these approaches are well known and will not be discussed herein beyond the approaches used in the embodiments described below.
Many conventional memory devices also use flash memory cells. Many conventional flash memory cells use floating gate technology to store one or more bits of information in the floating gate when program voltages are applied. The operation of floating gate flash memory devices is well known and will not be discussed herein for the sake of brevity. More recently, however, floating gate technology has been displaced by the use of other technologies that can be scaled to meet increasing memory density demands. For example, SONOS technology has become more prevalent in many applications. In a SONOS cell, the cell comprises a silicon layer (S), an oxide layer (O), a nitride layer (N), another oxide layer (O), and another silicon layer (S). A programming voltage applied to the SONOS stack causes a bit of data, or a charge, to be stored in the nitride layer. Then applying the appropriate read voltages to a SONOS cell, it can be determined whether the cell has been programmed.
While there have been advancements in conventional memory cell design, such as the development of PPROM and SONOS flash memory, new applications are constantly driving new memory requirements that cannot necessarily be met by the use of conventional memory structures. As such demands are likely to continue, and even increase, in the future, it is important to develop new techniques for memory structure design and fabrication.